1. Field of the Invention
The present invention relates to nylon compositions having improved surface aesthetics. More particularly, the invention pertains to nylon compositions of containing a nylon 6 homopolymer and a nylon-6,6,6 copolymer which has improved surface qualities while maintaining its physical properties. The invention also pertains to a method for preparing molded articles of from such nylon compositions. Nylons formed herein are useful to create injection molded articles having a good surface appearance and good performance at a low cost.
2. Description of the Related Art
It is known in the art to prepare blends of nylon homopolymers and copolymers. Typically, nylons are used to manufacture molded articles having good properties such as hardness, high tensile strength, toughness and aesthetics. The clarity of nylons, combined with the ease of molding them into articles of various shapes and surface textures, have resulted in many practical uses of nylons. However, when a nylon composition is modified to improve surface quality, the physical properties are often compromised. Properties such as cavity pressures, mold temperatures, cycle times and cooling times are affected, resulting in a greater production cost. Therefore, it has long been of interest to improve the appearance of a nylon while maintaining its strong physical properties.
Many attempts have been made to improve the various properties of a nylon composition through blending certain nylon homopolymers and copolymers. For example, U.S. Pat. No. 4,683,170 teaches a blend of nylon polymers which is used to form single and multilayer films. The nylon composition has low temperature extrusion processability and high heat resistance for heat sealing operations. U.S. Pat. No. 4,945,129 teaches certain polyamide compositions of which are combinations of nylon polymers and copolymer. The nylon composition has high impact strength and good toughness at low temperatures.
These references t each certain blends of nylon polymers, but do not teach the specific blend of a nylon 6 homopolymer with a nylon 6,6,6 copolymer having the characteristics of the present invention which include good surface appearance. The present invention solves the problems in the prior art by teaching a nylon composition useful for molding articles with good surface appearance and good performance at a low cost.
The invention provides a nylon composition comprising a substantially uniform blend of:
a) a first polyamide which is a nylon 6,6,6 copolymer formed from about 65 to about 75 weight percent of caprolactam and from about 25 to about 35 weight percent of hexamethylenediamine and adipic acid in a 1:1 ratio; and
b) a second polyamide which is nylon 6 homopolymer,
wherein the composition comprises from about 7 to about 20 weight percent of said first polyamide and from about 80 to about 83 weight percent of said second polyamide.
The invention also provides a nylon article comprising a substantially uniform blend of:
a) a first polyamide which is a nylon 6,6,6 copolymer formed from about 65 to about 75 weight percent of caprolactam and from about 25 to about 35 weight percent of hexamethylenediamine and adipic acid in a 1:1 ratio; and
b) a second polyamide which is nylon 6 homopolymer,
wherein the composition comprises from about 7 to about 20 weight percent of said first polyamide and from about 80 to about 83 weight percent of said second polyamide.
The invention further provides a method of forming a molded article comprising:
i) forming a substantially uniform, molten nylon composition comprising a mixture of
a) a first polyamide which is a nylon 6,6,6 copolymer formed from about 65 to about 75 weight percent of caprolactam and from about 25 to about 35 weight percent of hexamethylenediamine and adipic acid in a 1:1 ratio; and
b) a second polyamide which is nylon 6 homopolymer, wherein the composition comprises from about 7 to about 20 weight percent of said first polyamide and from about 80 to about 83 weight percent of said second polyamide; and
ii) molding said nylon composition to form an article.
In the present invention, a substantially uniform nylon composition is prepared by blending a first polyamide of a nylon 6,6,6 copolymer and a second polyamide of poly(caprolactam) (nylon 6) homopolymer. The nylon 6,6,6 copolymer (also sometimes referred to as nylon 6/66) is prepared through a condensation reaction of caprolactam, hexamethylene diamine and adipic acid by techniques well known in the art. Preferably, the nylon 6,6,6 copolymer comprises from about 65 to about 75 parts by weight of nylon 6 and from about 25 to about 35 by weight of nylon 6,6. More preferably, the nylon 6,6,6 copolymer comprises from about 68 to about 72 parts by weight of nylon 6 and from about 28 to about 32 parts by weight of nylon 6,6, and most preferably from about 69 to about 71 parts by weight of nylon 6 and from about 29 to about 31 parts by weight of nylon 6,6. This copolymer is also referred to as poly(caprolactamhexamethylene adipamide).
The nylon 6 homopolymer is prepared through a condensation of caprolactam by techniques well known in the art. The nylon 6,6,6 copolymer is prepared by condensing from about 65 to about 75 weight percent of caprolactam and from about 25 to about 35 weight percent of hexamethylenediamine and adipic acid in a 1:1 ratio. General procedures useful for the preparation of polyamides are well known to the art, and the details of their formation are described under the heading xe2x80x9cPolyamidesxe2x80x9d in the Encyclopedia of Polymer Science and Technology, published by John Wiley and Sons, Inc, Vol. 10, pps.487-491, (1969).
The polyamides used in the practice of this invention may be obtained from commercial sources or prepared in accordance with known preparatory techniques. For example, nylon 6 can be obtained from AlliedSignal Inc., Morristown, N.J. under the trademark CAPRON(copyright). Suitable variants of CAPRON(copyright) nylon resins for use in the present invention include CAPRON(copyright) nylon 8200, a balanced nylon 6 having a formic acid viscosity (FAV) of 75, CAPRON(copyright) nylon 8202, a balanced nylon 6 having an FAV of 45, CAPRON(copyright) nylon 1767, a balanced nylon 6 having an FAV of 35, CAPRON(copyright) nylon 8224HSL, a balanced nylon 6 containing a lubricant and a heat stabilizer, and having an FAV of 60, and CAPRON(copyright) nylon 1250, an amine-terminated nylon 6 with a FAV of 60.
The number average molecular weight of each polyamide homopolymer or copolymer may vary widely. Such are sufficiently high to form a free standing film but sufficiently low to allow melt processing of the blend. Such number average molecular weights are well known to those of skill in the film forming art and are usually at least about 5,000 as determined by gel permeation chromatography (GPC). Another measure of molecular weight is by the FAV method (ASTM D-789). This method provides a means of quickly determining molecular weight and can be comparable to the molecular weight as determined by the GPC method. In this method, a solution of 11 grams of aliphatic polyamide in 100 ml of 90% formic acid at 25xc2x0 C. is used. In the preferred embodiments of the invention, the number average molecular weight of each polyamide ranges from about 5,000 to about 100,000, and in the preferred embodiment it ranges between about 10,000 to about 60,000. Most preferred are those in which the number average molecular weight of each polyamide ranges from about 20,000 to about 40,000.
The weight percent of the first polyamide in the nylon composition ranges from about 7 percent to about 20 percent. More preferably the weight percent the first polyamide in the nylon composition ranges from about 10 percent to about 18 percent.
The weight percent of the second polyamide in the nylon composition ranges from about 80 percent to about 83 percent. More preferably the weight percent of the second polyamide in the nylon composition ranges from about 10 percent to about 18 percent.
The composition may be formed by dry blending solid particles or pellets of each of the polyamide components and then melt blending the mixture in a suitable mixing means such as an extruder, a roll mixer or the like. Alternatively, the composition may be formed by melting each of the polyamide compositions and then mixing them to form a substantially uniform nylon composition.
Typical melting temperatures range from about 175xc2x0 C. to about 260xc2x0 C., preferably from about 215xc2x0 C. to about 225xc2x0 C., and more preferably from about 220xc2x0 C. to about 223xc2x0 C. Blending is conducted for a period of time required to attain a substantially uniform blend. Such may easily be determined by those skilled in the art. If desired, the composition may be cooled and cut into pellets for further processing. For example, it may be extruded into a fiber, a filament, or a shaped element or it may be formed into films and optionally uniaxially or biaxially stretched by means well known in the art.
The compositions of this invention may optionally also include one or more conventional additives whose uses are well known to those skilled in the art. The use of such additives may be desirable in enhancing the processing of the compositions as well as improving the products or articles formed therefrom. Examples of such include: oxidative and thermal stabilizers, lubricants, mold release agents, flame-retarding agents, oxidation inhibitors, dyes, pigments and other coloring agents, ultraviolet light stabilizers, organic or inorganic fillers including particulate and fibrous fillers, reinforcing agents, nucleators, plasticizers, as well as other conventional additives known to the art. These conventional additives may be incorporated into compositions at any suitable stage of the production process, and typically are introduced in the mixing step and included in an extrudate. If included, a filler may comprise about 7 to about 50 weight percent but preferably 10 weight percent or less based on the weight of the overall composition.
By way of example, representative ultraviolet light stabilizers include various substituted resorcinols, sasalicylates, benzotriazole, benzophenones, and the like. Suitable exemplary lubricants and mold release agents include stearic acid, stearyl alcohol, stearamides. Exemplary flame-retardants include organic halogenated compounds, including decabromodiphenyl ether and the like. Suitable coloring agents including dyes and pigments include cadmium sulfide, cadmium selenide, titanium dioxide, phthalocyanines, ultramarine blue, nigrosine, carbon black and the like.
Representative oxidative and thermal stabilizers include the Period Table of Elements Group I metal halides, such as sodium halides, potassium halides, lithium halides; as well as cuprous halides; and further, chlorides, bromides, iodides. Also, hindered phenols, hydroquinones, aromatic amines as well as substituted members of those above mentioned groups and combinations thereof. Exemplary plasticizers include lactams such as caprolactam and lauryl lactam, sulfonamides such as o,p-toluenesulfonamide and N-ethyl, N-butyl benylnesulfonamide, and combinations of any of the above, as well as other plasticizers known to the art.
Suitable fillers include inorganic fillers, including those of fibrous and granular nature, as wells as mixtures thereof. The fibrous fillers include glass, silica glass, ceramic, asbestos, alumina, silicon carbide, gypsum, metal (including stainless steel) as well as other inorganic and carbon fibers. The granular fillers include wollastonite, sericite, asbestos, talc, mica, clay, kaolin, bentonite, and silicates, including alumina silicate.
Other granular fillers include metal oxides, such as alumina, silica, magnesium oxide, zirconium oxide, titanium oxide. Further granular fillers include carbonates such as calcium carbonate, magnesium carbonate, and dolomite, sulfates including calcium sulfate and barium sulfate, boron nitride, glass beads, silicon carbide, silicon as well as other materials not specifically denoted here. These fillers may be hollow, for example glass microspheres, silane balloon, carbon balloon, and hollow glass fiber.
Preferred inorganic fillers include glass fibers, carbon fibers, metal fibers, potassium titanate whisker, glass beads, glass flakes, wollastonite, mica, talc, clay, titanium oxide, aluminum oxide, calcium carbonate and barium sulfate. Particularly, glass fiber is most preferred. The inorganic fillers should preferably be treated with silane, titanate, or another conventional coupling agent, and glass fibers should preferably be treated with an epoxy resin, vinyl acetate resin or other conventional converging agent.
The invention also provides a nylon article comprising a substantially uniform blend of the first and second polyamides. The article is preferably formed into a molded article through a molding process, such as injection molding, which is well known in the art. An injection molding process softens the thermoplastic nylon blend in a heated cylinder, injecting it while molten under high pressure into a closed mold, cooling the mold to induce solidification, and ejecting the molded article from the mold.